Electrochromic devices, able to control their light transmittance, have been used in many different applications. Some examples are vehicle mirrors, construction windows, spectacles or different kinds of imaging devices. The transmittance is typically controlled by charging or discharging the electrochromic device. A typical electrochromic device comprises an electrochromic layered structure where an electrochromic layer and a counter electrode layer is separated by an electrolyte layer. By changing an electric potential between the electrochromic layer and the counter electrode layer, ions or electrons are caused to pass the electrolyte layer and the result is a changing or a discharging of the electrochromic layered structure, resulting in a colouring or bleaching of the electrochromic layer and/or counter electrode layer.
When a change of transmittance is requested, the charge of the electrochromic layered structure is to be changed. The change is typically requested to be performed as rapidly as possible, which is favoured by applying a high voltage across the electrochromic device. However, electrochromic devices may be damaged, or at least the life time of an electrochromic device may be significantly reduced is too high voltages are applied. It is therefore necessary to find a suitable compromise between transition time and applied voltages.
The optimum charging/discharging conditions for an electrochromic device may furthermore vary depending on the surrounding conditions, mainly the temperature. The optimum charging/discharging conditions for an electrochromic device may also vary from one application to another depending on e.g. the connections to the electrochromic layer. Differences in connection resistances will e.g. influence the optimum choice of charging/discharging conditions. Also, for one and the same device, the response to an applied voltage will change with time as the device becomes older. These effects are also typically more pronounced for large area devices.
In prior art, there are many different approaches for improving the transmittance control. In order to optimize the charging, it is common to use some kind of transmittance sensor and/or temperature sensor. The charging behaviour could then be adapted to the measured transmittance level/temperature. Some approaches utilizing actual measurements of transmittance and/or temperature are found in e.g. the U.S. Pat. No. 5,822,107, where a method is discloses that combines time control with measurements of physical characteristics such as voltage, current or light transmittance of the glazing. Preferably, also the temperature is measured by e.g. a thermocouple. In the published European patent application EP 2 161 615 A1 a process and apparatus for switching large-area electrochromic devices is disclosed. Presented conditions for charging/discharging are presented to be temperature dependent, which implicitly requires a temperature sensor to be available.
However, the inclusion of sensors increases the complexity and cost, and increases the risk that some assisting component may fail before the electrochromic film is worn out. The more components that are involved, the higher is the probability that some parts will malfunction. Furthermore, the sensors are often provided at a part of the window itself, either visibly from outside or hidden under some kind of frame. Transmittance sensors will typically occupy a part of the active area and temperature sensors also preferably are attached in the very vicinity of an active area, thereby reducing the available area. Moreover, mounting of sensors will add to the complexity and sensitivity for damage during mounting operations.
Other approaches for controlling charging/discharging of electrochromic devices utilizes electrical quantities. One example is illustrated in the published US patent application US 2004/0001056 A1. In the published U.S. Pat. No. 7,277,215 B2, a control system for electrochromic devices is disclosed. The document is primarily targeting electrochromic devices with solid-state electrolytes, in which leak currents is a problem. An open circuit voltage and a charging current are measured. By means of such measurements, temperatures or hysteresis curves can be estimated. It is however, not described how or even if optimum charging conditions can be deduced from such information. Similar control methods are disclosed also in the published US patent application US 2015/0070745 A1, also targeting electrochromic devices with solid-state electrolytes, in which leak currents are present.
A problem with prior art is that one wants to avoid use of external sensors, and still find suitable charging/discharging conditions for varying ages, configurations and surrounding conditions.